Our multifaceted investigation on normal and atypical melanocytes concerns the following topics: (1) biologically active 125I-MSH; (2) MSH-receptor hybridomas; (3) relative potencies of melano-tropins; (4) new regulators of melanin biosynthesis and the proliferation of melanoma cells; (5)\insulin and the control of melanoma cell proliferation; (6)\genetic and biochemical studies on pigmented hybrid cells that overexpress tyrosinase; (7)\internalization of MSH and fine structural localization of adenylate cyclase activity in response to MSH; (8) the growth of normal melanocytes in vitro.